The current epidemic of obesity has occurred despite the existence of [unreadable] a body weight regulatory system which, for most of mankind's history, [unreadable] has matched energy intake with energy expenditure sufficiently to avoid [unreadable] obesity. This suggests that the primary cause of the current obesity [unreadable] epidemic is not genetic, but may be due to an environment in which the [unreadable] energy balance regulatory system cannot function with sufficient [unreadable] precision to keep the population lean. In this application, we propose [unreadable] research aimed at understanding how factors in the environment, namely [unreadable] high fat/energy dense diets and physical inactivity, can promote obesity [unreadable] by affecting the precision of regulation of energy and fat balance. It [unreadable] is our intent to identify dietary and physical activity patterns that [unreadable] are associated with increased precision of energy balance regulation and [unreadable] which can prevent development of obesity. Laboratory data suggest that [unreadable] high fat diets promote obesity by increasing the probability of [unreadable] overconsumption of total energy. Our first aim is to systematically [unreadable] examine the relationship between dietary fat and energy intake across [unreadable] a range of diet compositions in sedentary subjects. While this has been [unreadable] done for diets with extreme variation in dietary fat (i.e., less than [unreadable] or equal to 20 percent vs greater than or equal to 40-60 percent) it has [unreadable] not been done for dietary fat content within the range of usual [unreadable] consumption of U.S. adults (i.e. 20-40 percent fat diets). We [unreadable] hypothesize that this relationship will not be linear and that there [unreadable] will be a threshold level or a range of dietary fat associated with [unreadable] a low probability of increased energy intake and positive energy [unreadable] balance. This information will be useful in developing dietary [unreadable] guidelines for obesity prevention. Our second aim is to determine how [unreadable] level of physical activity interacts with dietary fat content to affect [unreadable] the likelihood of developing positive energy balance. We hypothesize [unreadable] that the optimum level of dietary fat to minimize the probability of [unreadable] positive energy balance will depend on level of physical activity and [unreadable] the optimum level of physical activity to minimize the likelihood of [unreadable] positive energy balance will vary with dietary fat content. This work [unreadable] will be among the first to study the interaction of dietary and physical [unreadable] activity patterns in promotion and prevention of obesity. The results [unreadable] will help identify the changes required in current dietary and physical [unreadable] activity patterns if we are going to be successful in preventing the [unreadable] development of obesity. [unreadable]